1. Field of the Invention
This invention relates generally to foamed polymer products. More particularly, the invention concerns a chemical foaming agent and a method of using such agent to make molded foamed polymer articles.
2. Description of Related Art
Foaming agents, also known as blowing agents, are used during processing of polymers to achieve a cellular structure in the final product. As described in the article "Foaming Agents" in Modern Plastics Encyclopedia (Mid-October 1989 Issue Volume 66, Number 11), chemical blowing agents (CBAs) are substances that decompose at elevated temperatures or through chemical reaction and form gaseous composition products. Reasons given for the use of foaming agents include weight reduction for savings in cost and material, better properties such as insulation against heat or noise, different surface appearances, improved stiffness, and different electrical properties.
One widely used inorganic chemical blowing agent is sodium borohydride ("SBH"). As explained in Lapierre et al. U.S. Pat. No. 4,769,397, SBH reacts with water or some other proton donor or "activator" to release gas for foaming. That patent describes an activation system comprising a mixture of sodium bicarbonate and citric acid which reacted to evolve water and carbon dioxide.
In some applications the use of a sodium bicarbonate/citric acid activation system has been associated with a phenomenon known as "chalking," in which a white material believed to be borax and sodium carbonate forms on the surface of a molded part. One object of this invention is to provide an improved activator for SBH to avoid chalking problems.
Commercially available foaming agents are typically in the form of concentrates: active ingredients encapsulated in a carrier resin. Another object of this invention is to provide an improved foaming agent concentrate which is stable during storage.